Implantable medical devices are commonplace today, particularly for treating cardiac dysfunction. Cardiac pacemakers, for example, are implantable medical devices that replace or supplement a heart's compromised ability to pace itself (i.e., bradycardia) due to chronotropic incompetence or a conduction system defect by delivering electrical pacing pulses to the heart. Implantable cardioverter/defibrillators (ICD's) are devices that deliver electrical energy to the heart in order to reverse excessively rapid heart rates (tachycardia) including life threatening cardiac arrhythmias such as ventricular fibrillation. Since some patients have conditions that necessitate pacing and also render them vulnerable to life-threatening arrhythmias, implantable cardiac devices have been developed that combine both functions in a single device.
Most pacemakers today are operated in some sort of synchronous mode where the pacing pulses are delivered in a manner that is dependent upon the intrinsic depolarizations of the heart as sensed by the pacemaker. ICD's must also sense the electrical activity of the heart in order to detect an arrhythmia that will trigger delivery of the shock pulse in an attempt to reverse the condition. Such sensing information can be used to initiate another mode of therapy, and efforts have been made in the past to combine automatic drug delivery by an implantable drug delivery system with either pacemakers, ICD's, or both in order to treat cardiac arrhythmias.
Implantable drug delivery systems suffer from a number of disadvantages, however, when compared with an external drug delivery device. Although the drug reservoir of an implantable delivery device can be replenished, it is difficult to change the drug once it is put into the reservoir, making patient management difficult in cases where a patient's condition either changes or otherwise requires a change of medication. In addition, drugs degrade over time. Finally, there is the risk of leakage from the implanted reservoir, the consequences of which can range from an annoyance to a medical emergency. For these reasons, drug delivery from an external device is preferred in many situations.
In order to control the delivery of drugs or other therapies by an external device, the implanted device must be capable of transmitting command and control information to the external device. This is the primary problem with which the present invention is concerned.